The European Telecommunications Standard Institute—Digital Mobile Radio (ETSI-DMR) is a direct digital replacement for analog Private Mobile Radio (PMR). DMR is a scalable system that can be used in unlicensed mode (in a 446.1 to 446.2 Megahertz (MHz) band), and in licensed mode, subject to national frequency planning.
DMR promises improved range, higher data rates, more efficient use of spectrum, and improved battery. Features supported include fast call set-up, calls to groups and individuals, short data and packet data calls. The communications modes include individual calls, group calls, and broadcast calls and, provided via a repeater mode or a direct communication mode among the radios operating within the network.
Other important DMR functions such as emergency calls, priority calls, full duplex communications, short data messages and Internet Protocol (IP)-packet data transmissions are supported.
Direct mode is a mode of operation where radios may communicate outside the control of a network. A radio, as used herein, can be mobile and/or fixed end equipment that is used to obtain DMR services. Direct mode is a communication technique where any radio can communicate with one or more other radios without the need for any additional infrastructure equipment (i.e. repeaters). Direct mode operation is in contrast to the conventional repeater mode which is a mode of operation where radios communicate through infrastructure equipment such as a repeater. Direct mode, therefore, provides more efficient, less costly communication system operation.
The European Telecommunications Standard Institute—Digital Mobile Radio (ETSI-DMR) standard (ETSI TS 102 361-1) describes a time division multiple access (TDMA) air interface protocol. Any of the ETSI standards or specifications referred to herein may be obtained by contacting ETSI at ETSI Secretariat, 650, route des Lucioles, 06921 Sophia-Antipolis Cedex, FRANCE. The Digital Mobile Radio (DMR) air interface standard includes a specification of a protocol used by radios that can both transmit and receive signals at the data link layer (i.e., layer 2) of the well known seven-layer Open Systems Interconnection computer networking model. The ETSI DMR standard specifies a two-slot TDMA structure that transmitting and receiving radios can utilize to send voice and/or data signals. The voice and data signals are transmitted in the TDMA slots in accordance with a general burst format specified in the standard.
The ETSI-DMR standard (ETSI TS 102 361-1) also provides for direct mode transmissions. Channel access, according to the ETSI-DMR standard, further requires accommodation of different levels of radio “politeness” (i.e. Listen Before Transmit (LBT)) and accounting for the co-existence with analog activity and other digital protocols on the same radio frequency (RF) carrier.
The ETSI DMR standard defines channel access for direct mode. For example, ETSI TS 102 361-1 V1.4.5 (2007-12) 5.2.1.1 describes the monitoring by a DMR entity of a channel received signal strength indicator (RSSI) level to determine whether activity is present on a channel. If after a maximum period of time the RSSI level has not exceeded a configurable (within a predefined range) threshold, then the DMR entity presumes that activity is not present on the channel. If however the RSSI level does exceed the threshold, then the DMR entity presumes that activity is present on the channel and it attempts to become synchronized to the activity for specific channel access policies. However, these channel access mechanisms are spectrally inefficient because only one direct mode communication path is provided on the RF channel.
Accordingly, there is a need for a method and apparatus for spectrally efficient channel access rules for TDMA direct mode.
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The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.